This invention concerns the field of filtration and more specifically, filtration using rotary discs and filters.
Filtration devices are used to separate one or more components of a fluid from other components. Common processes carried out in such devices include classic filtration, microfiltration, ultrafiltration, reverse osmosis, dialysis, electrodialysis, pervaporation, water splitting, sieving, affinity separation, affinity purification, affinity sorption, chromatography, gel filtration, and bacteriological filtration. As used herein, the term "filtration" includes all of those separation processes as well as any other processes using a filter that separate one or more components of a fluid from the other components of the fluid.
Filtration processes make use of the greater filter permeability of some fluid components than others. As used herein, the term "filter" includes any article made of any material that allows one or more components of a fluid to pass through it to separate those components from other components of the fluid. Thus, the term "filter" includes metallic and polymeric cloth filters, semipermeable membranes and inorganic sieve materials (e.g., zeolites, ceramics). A filter may have any shape or form, for example, woven or non-woven fabrics, fibers, fiber bundles, membranes, sieves, sheets, films, rods, and combinations thereof.
The components of the fluid that pass through the filter comprise the "permeate" and those that do not pass (i.e., are rejected by the filter or are held by the filter) comprise the "retentate." The valuable fraction from the filtration process may be the retentate or the permeate or in some cases both may be valuable.
A common problem in all filtration devices is blinding or clogging of the filter. Permeate passing through the filter from the fluid layer adjacent to the feed side of the filter leaves a retentate layer adjacent to or on that side of the filter having a different composition than that of the bulk feed fluid. This material may bind to the filter and clog its pores (that is, foul the filter) or remain as a stagnant boundary layer, either of which hinders transport of the components trying to pass through the filter to the permeate product side of the filter. In other words, mass transport through the filter per unit time (i.e., flux) is reduced and the inherent sieving capability of the filter is adversely affected.
Generally, fouling of the filter is chemical in nature, involving chemisorption of substances in the feed fluid onto the filter's internal (pore) and external surface area. Unless the chemical properties of the filter surface are altered to prevent or reduce adsorption, frequent and costly filter replacement or cleaning operations are necessary.
One of the most common causes of fouling arises from the low surface energy (e.g., hydrophobic nature) of many filters. U.S. Pat. Nos. 4,906,379 and 5,000,848, which are commonly assigned with this application, disclose chemical modification to increase the surface free energy (e.g., hydrophilicity) of filter surfaces. In general, however, relatively little attention has been given to modifying surface chemistry to reduce filter fouling. (U.S. Pat. Nos. 4,906,379 and 5,000,848 and all of the other patent documents, publications, and other documents cited in this application are incorporated herein by reference in their entireties for all purposes.)
In contrast to the chemical nature of most fouling problems, the formation of a boundary layer near the surface of the filter is physical in nature, arising from an imbalance in the mass transfer of feed fluid components towards the filter surface as compared to the back-transfer from the boundary layer to the bulk feed fluid. Some form of force (for example, mechanical, electro-kinetic) must be used to promote the desired mass transfer away from the filter surface. Unfortunately, few strategies have been developed that promote adequate back-mixing to reduce the boundary layer or prevent its formation.
The most common strategy is called "cross-flow" filtration ("CFF") or "tangential flow" filtration ("TFF"). In principle, the feed fluid is pumped across (i.e., parallel to) the outer surface of the filter at a velocity high enough to disrupt and back-mix the boundary layer. In practice, however, cross-flow has several disadvantages.
An inherent weakness common to all TFF systems is that a significant pressure drop exists from feed inlet to feed outlet. This pressure drop, which is due to the high feed rate and the narrow feed channel required to improve filtration efficiency, causes the transmembrane pressure ("TMP") across the filter to be non-uniform. In fact, this non-uniformity in TMP is so great that in most instances the average TMP is sufficiently high to cause fouling of the membrane. This problem worsens when the feed rate is increased in an attempt to avoid this fouling because the non-uniformity in TMP increases with increased feed flow rate. The close coupling of the two key operating parameters that control filtration efficiency (TMP and feed rate) makes it difficult, if not impossible, to optimize and precisely control the filtration process.
Additionally, this makes scaling up TFF systems difficult. Many systems are scaled-up by adding membrane modules (and, thus, area) in series. Because that increases the fluid flow path length, that approach results in even higher pressure drops along the feed path and induces greater non-uniformity in TMP. The net result is greater unpredictability in performance as the scale increases.
Finally, many substances in process fluids can not withstand the high shear rates associated with the necessary high flow rates. For instance, the maximum allowable shear and/or velocity for many biological fluids are far too low to allow adequate back-mixing to reduce or eliminate the stagnant boundary layer. Furthermore, the required high feed rates as compared to the filtration rates require numerous feed recycles through the system, which are also undesirable. Thus, TFF is less desirable or cannot be used at all in many cases.
A different approach to eliminating the stagnant boundary layer involves decoupling the feed flow rate from the applied pressure. With this approach, a structural element of the filtration device, rather than the feed fluid, is moved to effect back-mixing and reduction of the boundary layer. The moving body may be the filter itself or a body located near the filter element.
The moving-body approach to enhance filtration often involves turbulent fluid flow (e.g., from the use of rotating and/or vibrating discs). Unfortunately, turbulent flow is energy inefficient and may damage delicate forms of matter (e.g., biologicals).
One of the rare moving-body devices that has enhanced filtration without turbulence is exemplified in U.S. Pat. No. 4,790,942 (commonly assigned with this application). This patent discloses the use of a filtration apparatus comprising outer and inner cylindrical bodies defining an annular gap for receiving a feed fluid. The surface of at least one of the bodies defining the gap is the surface of a filter, and one or both of the bodies may be rotated. Induced rotational flow between these cylinders is an example of unstable fluid stratification caused by centrifugal forces. The onset of this instability can be expressed with the aid of a characteristic number known as the Taylor number. Above a certain value of the Taylor number, a vertical flow profile comprising so-called Taylor vortices appears. This type of secondary flow causes highly efficient non-turbulent shear at the filter surface(s) that reduces the stagnant boundary layer thickness and, thus, increases the permeate flux.
In contrast to classic cross-flow filtration, the device of U.S. Pat. No. 4,790,942 allows the shear rate near the filtration surface and the transmembrane pressure to be independently controlled. Furthermore, because those two operating parameters are independent and high feed rates are not required to improve the permeate flux, the feed rate can be adjusted to avoid non-uniform transmembrane pressure distributions. Accordingly, mechanically agitated systems of this type enable precise control over the separation.
Taylor flow is only one example of instability associated with flow along curved walls. Flow along concave walls also may cause a similar kind of instability and can lead to a secondary flow characterized as Taylor-Goertler vortices. Transverse double toroidal secondary flow referred to as Dean flow is also known (see, for example, U.S. Pat. No. 4,311,589, particularly FIG. 2 and column 2, lines 3-17, and column 3, lines 9-25). U.S. Pat. No. 4,592,848 employs helical flow. A complex flow profile also exists within the gap between a rotating sphere within a stationary outer sphere in contrast to the uniform arrangement of Taylor vortices within the annular gap formed by rotating and stationary cylinders. With that two-sphere system, different flow regimes occur simultaneously side by side, and flow at the equator may be vortical but not at the poles. Non-uniform flow instabilities may also occur when rotating a disc in a fluid, and flow profiles typically have both purely laminar and turbulent regions. Under certain conditions, rotating a flat disc in a stationary fluid can develop flow profiles with regions of stationary vortices that assume the shape of logarithmic spirals. In general, flow profiles associated with rotating discs depend highly upon radial position and are quite non-uniform like those with concentric spheres and unlike those with coaxial cylinders. As discussed below, the lack of flow uniformity with rotating discs makes them undesirable for filtration applications because the lack of uniformity causes energy and other inefficiencies.
A number of filter systems using mechanical agitation currently exist. In U.S. Pat. No. 4,376,049 a circular design is employed in the form of a rotary apparatus. Centrifugal forces cause permeate to flow from the center of the rotor plate through the rotating filter elements. The vanes on the rotor plate holding the filter elements are curved rearwardly with respect to the forward direction of rotation. The curvature helps retain the filter elements in place and enhances the pumping capability of the system as the rotor is rotated in the forward direction. The filter elements eventually become clogged or packed with material and must be replaced.
In U.S. Pat. No. 3,477,575 one set of filter elements is mounted on a common shaft for rotation. A second set of filter elements is fixed on the inner wall of the device, and the rotatable and stationary filter elements are interleaved to create a serpentine flow path along which the feed fluid flows. Permeate is withdrawn through both sets of filters.
This type of filtration device was commercialized by the Swiss company Willi Bachofen. Because of the interleaving of the two sets of filter elements and because of the location of the feed fluid inlet and outlet, a substantial portion of the feed fluid flow is generally parallel to the major surfaces of the filters. Thus, although this device uses rotation of mechanical elements (one set of filters), it also uses cross-flow to help reduce stagnation at the fluid/filter interface.
Historically, such rotary filters have been used at relatively low rotation rates to dewater suspensions in so-called cake filtration (see Schweigler and Stahl, "High Performance Disc Filter for Dewatering Mineral Slurries," Filtration and Separation, January/February, pages 38-41 (1990)). Mass transfer from filter surface to feed fluid is poor in such cases, hence the build-up of a cake upon the filter. The thickness of such cakes can be limited by using scraper blades near the filter surface (see Ingersoll-Rand, "Upgrade your entire filtering and/or washing operation with the new Artisan Dynamic Thickener/Washer," Bulletin No. 4081, 4 pages (2/86); and Ingersoll-Rand, "Patented filter/wash capability permits simultaneous washing and filtering," Bulletin No. 4060, 4 pages (8/83)).
Another strategy for reducing the stagnant boundary layer adjacent to a rotating filter surface is to use high rotation rates, e.g., 1000 rpm. See Parkinson, "Novel Separator Makes Its Debut," Chemical Engineering (Jan. 1989), 1-page reprint by Aqua Technology Resource Management, Inc.; Aqua Technology Resource Management, Inc., "How to Keep Your Fluid Processing Budget from Going to Waste," 3-page brochure; and Aqua Technology Resource Management, Inc., 4-page brochure (untitled) discussing "Technology Background," "Overcoming Concentration Polarization," etc. However, use of a rotating filter element is complicated by the need for adequate rotary sealing means to maintain the separation of feed fluid from permeate. Also, the rotating filter and sealing means need to be strong enough to withstand centrifugal forces.
An alternative disc filter design utilizes a stationary filter and a closely spaced non-filter rotating element. Wronski, Molga, and Rudniak, "Dynamic Filtration in Biotechnology," Bioprocess Engineering, Volume 4, pages 99-104 (1989), report testing such a device against a device that has a stationary filter and an oppositely disposed rotating disc filter, and against a cross-flow device, and against a rotating cylindrical filtration device. In one comparison, the steady state flux through a rotating disc filter was about four times the flux through a stationary disc filter. In another comparison, the rotating cylindrical filter had the highest flux and the cross-flow device had the lowest flux and the highest energy consumption. In a third comparison, the rotating cylindrical filter had a flux over 30% higher than the flux in the stationary filter/rotating non-filter element device. See also Wronski and Mroz, "Power Consumption in Dynamic Disc Filters," Filtration & Separation, November/December, pages 397-399 (1984); Wronski, Rudniak, and Molga, "Resistance Model for High-Shear Dynamic Microfiltration," Filtration & Separation, Nov./Dec., pages 418-420 (1989 ); Rudniak and Wronski, "Dynamic Microfiltration in Biotechnology," Proceedings 1st Event: Bioprocess Engineering, Institute of Chemical and Process Engineering, Warsaw University of Technology, Warsaw, Poland, Jun. 26-30, 1989; Molga and Wronski, "Dynamic Filtration in Obtaining of High Purity Materials--Modelling of the Washing Process," Proceedings of the Royal Flemish Society of Engineers, Antwerp, Belgium, Oct. 1988, Volume 4, pages 69-77; Wronski and Mroz, "Problems of Dynamic Filtration," Reports of the Institute of Chemical Engineering, Warsaw Techn. Univ., T.XI, z.3-4, pages 71-91 (1982); and Wronski, "Filtracja dynamiczna roztworow polimerow," Inz. i Ao. Chem., number 1, pages 7-10 (1983).
Murkes and Carlsson, Crossflow Filtration--Theory and Practice, 133 pages, John Wiley & Sons, New York (1988), particularly pages 22-26 and 69-125 and most particularly in Section 3.5 at pages 93-100, disclose that the flux of a stationary filter can be enhanced by rotating various elements near the stationary filter. The elements reported to have been tested include a cross, a double cross, a propeller, a spoke wheel, and discs either flat (plain) or having radial blades (vanes extending from the plane of the disc). According to Murkes and Carlsson, the flat disc was the least efficient. The increase in flux resulting from use of the other elements was accompanied by increased power consumption. That is consistent with findings by Watabe, "Experiments on the Fluid Friction of a Rotating Disc with Blades," Bulletin of JSME, Volume 5, number 17, pages 49-57 (1962), concerning the fluid friction of rotating discs that have protuberances or projections (i.e., blades) on their surfaces, including radial, concentric, and spiral blades, e.g., see the drawings of discs with backward and forward spiral blades at page 54.
Shirato, Murase, Yamazaki, Iwata, and Inayoshi, "Patterns of Flow in a Filter Chamber during Dynamic Filtration with a Grooved Disk," International Chem. Eng., Volume 27, pages 304-310 (1987), describe a study to determine the effects on flow patterns of, among other things, the presence and dimensions of radial grooves in a rotating disc filter. The experimental apparatus had a stationary disc filter and a closely spaced rotating plate above it (see, e.g., FIG. 1 at page 305) but the study was carried out without actual filtration. The grooves provided significantly greater radial velocities and tangential velocities to the liquid in the filter chamber as compared to an ungrooved (i.e., flat) rotating disc and were beneficial in sweeping filter cake out of the filter chamber. A disc with eight radial grooves had a tangential fluid velocity nearly twice that of an ungrooved rotating plate. However, Watabe, "Experiments on the Fluid Friction of a Rotating Disc with Blades," Bulletin of JSME. Volume 5, number 17, pages 49-57 (1962), shows that increases in the number of radial blades beyond eight had little beneficial effect and that substantially more than eight reversed the beneficial effects and caused the rotating disc to behave as if it were a flat plate.
Accordingly, there is a need for mechanically agitated disc filters that, among other things, have reduced energy requirements and higher fluxes, that minimize the adverse effects of the filtration process (e.g., shear) on the feed liquid, retentate, and permeate, that have relatively simpler construction, that can be scaled up relatively easily, and that make the most efficient use of space in a process plant.